1. Field of the Invention
The present invention relates to a braking force retaining unit which can retain a braking force even after the depression of a brake pedal has been released.
2. Description of the Background Art
A braking force retaining unit is configured to include an electromagnetic or solenoid valve which operates electromagnetically at an intermediate position along the length of a brake hydraulic circuit so as to cut off the brake hydraulic circuit by supplying the solenoid valve with an electric current, whereby even after the driver releases the depressed brake pedal, brake hydraulic pressure (braking force) is retained at the wheel cylinders. As a result, the vehicle is, for example, prevented from moving backward from its own weight when attempting to start climbing up a rising slope. Then, in the event that the retained brake hydraulic pressure is made zero or made to collapse totally at one time, occupants of a vehicle have to be made to feel a feeling of some abruptness or physical disorder, which is not preferable from the viewpoint of product quality. Due to this, in JP-A-2001-354126 (FIGS. 4, 8, 12 and the like), when releasing a retained brake hydraulic pressure, a time difference is provided in release initiating timing between two crossed brake hydraulic pressure circuits (crossed brake pipe lines).
However, in order to enable a smoother start, the feeling of abruptness and physical disorder that the occupants of the vehicle are forced to feel needs to be reduced further. In addition, in the case of JP-A-2001-354126, in the event that a large time difference is set in release initiating timing between two crossed brake hydraulic pressure circuits, since a behavior such as one in which a body of the vehicle is twisted in some way is generated in the crossed brake pipe lines, there is imposed a limitation on setting of time. Hence the compatibility of the reduction in a shock felt by the occupants when the applied brakes are released with the reduction in the behavior in which the body of the vehicle is twisted needs a certain compromise.